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Old 02-19-2009, 06:30 AM   #1
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Exterior Paint after Rain

How long should I wait to paint after a good rain storm?

It rained most of the day yesterday, and today it's 96% humidity. I am supposed to paint 8 shutters and a front door today. I just don't want to run into problems with peeling down the road.

Shutters are plastic, and door is metal or fiberglass.



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Old 02-19-2009, 09:37 AM   #2
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I would think that this would vary by paint. I would call the tech support line for the product and ask them. Certainly the recoat time is going to be a LOT longer with that kind of humidity.



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Old 02-19-2009, 10:09 AM   #3
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In my view, it all depends on what kinda paint you're wanting to use.

I think you could at least apply an exterior alkyd primer while you're waiting for the humidity to subside. And, if it doesn't subside, you could just apply an alkyd paint instead.

Latex paint is a whole nuther can of worms.

Latex paints consist of a SLURRY of coloured solid particles called pigments and clear hard plastic particles called resins all suspended in a mixture of water and a low volatility water soluble solvent called a coalescing agent.

When you apply latex paint, the water evaporates from the wet paint film leaving the clear hard plastic particles surrounded by an ever increasing concentration of that coalescing solvent. It's that coalescing solvent that softens those clear hard resins into soft mushy blobs so that they stick to and pull on one another to form a continuous plastic film with the pigments suspended inside it much like raisins in raisin bread.

Over the next 2 or 3 day, the coalescing solvents evaporate from the paint film (giving you that "freshly painted smell") and the plastic hardens back up to it's original hardness.

If you paint in high humidity like 96%, and it stays that way for 2 or 3 days, the coalescing solvents will be evaporating from the paint faster than the water will (cuz of the high humidity). The result will be that the paint will never have a sufficient concentration of those coalescing solvents in it, and the plastic binder resins will never soften up sufficiently to form a continuous plastic film. Instead, you'll get a clowdy powdery film on the wall (or doors or shutters or whatever) instead of a coat of paint.

Normally you can get away with painting after it rains because the weather quickly improves so that the normal order of things is restored, with water evaporating into the air faster than those coalescing solvents. But, you do so at your own peril because if it stays wet and rainy, your paint may very possibly be wrecked.

Also, I think an exterior alkyd paint could be applied to DRY wood shortly after a rain without it affecting the paint film. It takes a while for the wood to absorb moisture from the humidity in the air (certainly more than one or two days), so the dimensions of the wood wouldn't have changed significantly. And, exterior alkyd paints are soft and elastic enough to accomodate changes in wood's dimensions.

In this case, if you're painting over plastic, fiberglass or metal, none of those materials change dimensions with humidity, so I'd say your best bet might be to just go with an alkyd primer and alkyd paint on this one if you have to do the job right away.
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Old 02-19-2009, 06:47 PM   #4
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As neither vinyl, metal, or fiberglass hold/absorb moisture, any, when wiped dry after rain, would be OK rather quickly
However, 96% humidity is more than most coatings would care for
Check the product's tech sheet, it'll say "90/80/70%" or whatever
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Old 02-19-2009, 11:49 PM   #5
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I don't think I would paint with anything in 96% humidity. You could trap moisture under the paint if you use oil and if you use latex you can inhibit it's ability to dry properly.
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Old 02-20-2009, 04:37 AM   #6
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96% humidity.

Where in the world do you live? In the jungle?
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Old 02-20-2009, 06:08 PM   #7
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Originally Posted by chrisn View Post
96% humidity.

Where in the world do you live? In the jungle?
Ha ha haaa...Inland Boy!

Not that it's often, but we out here in the oceans (by no means all jungle...or even warm-I'm in the North Atlantic) can experience days or weeks of over 90%

I've also worked on North American peninsulas in technically semi-tropic zones (FLA), that have had the same

Admittedly, 96% is up there...but not out of the realm of possibility by any means
(and it's very rare that I can "bust your stones" so to speak...lol...sorry man)


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